Simple uname(2) wrapper to work around a CrashPlan bug and enable real-time watching

crashplan_uname.c

/*
 * Simple uname(2) wrapper library that appends ".0" to any release version
 * of the form X.Y -- for example, "3.16-2-amd64" becomes "3.16.0-2-amd64".
 *
 * This crude hack is meant to work around a CrashPlan bug[*]: real-time
 * watching is only enabled after checking the kernel version for inotify
 * support (which was added in 2.6.13), but the version is expected to be of
 * the form X.Y.Z.  If this is not the case, CrashPlan assumes that inotify
 * is not supported, and real-time watching is disabled.
 *
 *  [*] https://bugs.debian.org/745984
 *
 *
 * INSTRUCTIONS:
 *
 * 1. Compile (feel free to rename the .so):
 *
 *  gcc -Wall -O2 -shared -fPIC -ldl crashplan_uname.c -o libcrashplan_uname.so
 *
 * 2. (Optional) Install the .so where it can be found by ld.so:
 *
 *  mv libcrashplan_uname.so /usr/local/lib/  &&  ldconfig
 *
 * 3. Add the following line to CrashPlan's bin/run.conf:
 *    (You will need to provide a full pathname if you skipped step 2.)
 *
 *  export LD_PRELOAD=libcrashplan_uname.so
 *  
 * You can now (re-)start the CrashPlan engine.  Enjoy!
*/

#define _GNU_SOURCE  /* RTLD_NEXT is a GNU feature */
#include <dlfcn.h>  /* dynamic linking */
#include <sys/utsname.h>  /* uname() itself */

#include <ctype.h>  /* isdigit() */
#include <string.h>
#include <stdio.h>
#include <stdlib.h>


int uname (struct utsname *buf) {
	/* real uname() function */
	static int (*real_uname) (struct utsname *buf);
	int ret;
	/* dlsym() error, if any */
	char *error;
	/* We'll need to scan the release string, and maybe copy part of it */
	char *p, *tmp;
	int n_dots = 0;

	/* Find the real uname() if we haven't already written it down */
	if (!real_uname) {
		real_uname = dlsym(RTLD_NEXT, "uname");
		if ((error = dlerror()) != NULL) {
			fprintf(stderr, "%s\n", error);
			exit(1);
		}
	}

	ret = real_uname(buf);

	/* No need to continue if there's not enough room for ".0" anyway */
	if (strlen(buf->release) + 2 >= sizeof(buf->release))
		return ret;

	/* Counting the dots... (the field is guaranteed null-terminated) */
	for (p = buf->release; *p; p++) {
		if (*p == '.')
			n_dots++;
		else if (! isdigit(*p))
			break;
	}

	if (n_dots < 2) {
		/* (No need for strndup/strncpy -- we know everything fits) */
		tmp = strdup(p);
		*p = '.'; p++;
		*p = '0'; p++;
		strcpy(p, tmp);
		free(tmp);
	}

	return ret;
}